The present invention relates to a non-linearity compensating circuit for high-frequency amplifiers, and more particularly to a non-linearity compensating circuit for high-frequency amplifiers having a non-linearity characteristics with respect to the input amplitude vs. output amplitude relationship and input amplitude vs. output phase relationship. Since the compensating circuit according to the present invention is compact and inexpensive, and has a reduced power loss so that it can be constructed in a microwave integrated circuit form, it is useful for on-board power amplifiers of the communication satellites, high-power transmitters for ground stations, and wide-band transistor amplifiers. The circuit according to the present invention allows these high-frequency amplifiers to operate efficiently in their compensated linear regions.
It is generally recognized that the negative feedback technique provides the best method for compensating a non-linearity of an amplifier and this technique has been widely used to compensate the non-linearity of ordinary amplifiers, but it is extremely difficult to apply this technique to very high-frequency amplifiers. It has been recognized that an employment of a pre-distorter is most adapted for high power and high frequency amplifiers; the pre-distorter being placed in front of the high-frequency amplifier to be compensated, has a non-linearity characteristics that is just the opposite of the non-linearity characteristics of the high-frequency amplifier to be compensated so that the overall characteristics of the combination becomes linear.
FIG. 1 shows an example of the conventional pre-distorters consisting of a branch coupler 2, a pair of channels consisting of the first channel L.sub.1 and the second channel L.sub.2, and a signal synthesizing circuit 5, said branch coupler 2 divides the input signal on the input terminal 1 into two components, said first channel L.sub.1 includes a high-frequency amplifier for generating non-linear distortion and receives one of the two signal components from said branch coupler 2, said second channel L.sub.2 includes a delay line 4a and receives the other of the two signal components from said branch coupler 2 and said synthesizing circuit 5 takes the vectorial sum of a part of the signal from the amplifier 3 and a part of the signal from the delay line 4a. A signal voltage e.sub.T, which is a part of the output signal e.sub.T of the high frequency amplifier 3 and includes an amplitude and phase distortion having originated from the non-linearity of the amplifier 3, and a voltage signal e.sub.R, which is a part of the output signal e.sub.R through the delay line 4a, are summed vectorially in an antiphase relationship in the pre-distorter so that the pre-distorter provides an input/output characteristics which is just the opposite of the input/output characteristics of the high-frequency amplifier to be compensated.
In order to take the vectorial sum of the two signals in an antiphase relationship it is essential that the effective electric length of the first channel L.sub.1 having a non-linear distortion generator 3 and the effective electric length of the second channel L.sub.2 having a delay line 4a are approximately equal. This means that if a low power, low noise and high gain travelling wave tube (TWT) amplifier were used as the non-linear distortion generator 3 and a coaxial cable were used as the delay line 4a, the pre-distorter circuit would have a larger volume and higher power loss since the second channel length would have to be long in order to compensate the delay in the TWT amplifier which is long by nature. It might appear possible that the above-mentioned trouble would be elliminated if a high-frequency transistor amplifier were used as the distortion generating circuit 3 since a transistor amplifier has a shorter delay. But in this case, since the gain of a one-stage transistor amplifier is small, one would have to set the branching ratio of the branch coupler 2 of FIG. 1 so that the signal power applied to the high-frequency transistor amplifier is much larger than the signal power applied to the delay line 4a, or one would have to use a multi-stage transistor amplifier in order to obtain a sufficiently high gain. Thus, the employment of a one-stage transistor amplifier results in a reduction of the input signal fed to the delay line causing a larger power loss in the compensation circuit, and the employment of a multi-stage transistor amplifier has a disadvantage that a broad-band characteristics is difficult to obtain with a multi-stage transistor amplifier. Moreover, the conventional pre-distorter requires variable attenuators and variable phase-shifters for adjusting the amplitude and the phase relationship between the two output signals from the first channel L.sub.1 and the second channel L.sub.2 before these signals are vectorially summed at the synthesizing circuit 5 in order to obtain an output signal that cancels the distortion of the high frequency circuit to be compensated.
It is, therefore, another disadvantage of the conventional configuration that the pre-distorter circuit has a large volume and accompanies a large power loss due to the introduction of the variable attenuator and the variable phase shifter.
Thus, the pre-distorter circuit shown in FIG. 1 has several disadvantages. For one thing, the size, shape and the transmission loss of the delay line 4a present great problems when the circuit is to be constructed in a microwave integrated circuit (MIC) form. Secondly, it is very difficult to make a compact circuit having a low power-loss according to the prior technique because the variable attenuator and the variable phase-shifter have a large volume and a large power loss.